Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2006 March 29

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Green and Black Aurora Over Norway
Credit & Copyright: Frank Andreassen (

Explanation: What causes gaps between aurora curtains? These unusual gaps can make auroral displays appear more detailed and intricate. Research using data from four Cluster spacecraft orbiting the Earth has likely found the secret: auroral gaps, sometimes knows as black auroras, are actually anti-auroras. In normal auroras, electrons and/or predominantly negatively charged particles fall toward Earth along surfaces of constant magnetic field. They ionize the Earth's atmosphere on impact, causing the bright glows. In auroral gaps, however, negatively charged particles may be sucked out from the Earth's ionosphere along adjoining magnetic field lines. These dark anti-auroras can climb to over 20,000 kilometers and last for several minutes. Pictured above, a series of well-defined auroral gaps is seen dividing green aurora curtains high above Harstad, Norway, earlier this month.

News: Solar Eclipse Today
Tomorrow's picture: orbiting Mars

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